No matter how careful you are making a tool, something will be out of alignment. Especially after you smack the tool a few times. Also, your dominant hand/side will fight your eyes when trying to put a work piece square to an anvil or power hammer. (looks like Joel is left handed from the way he set up his PH dies.) Drag from tooling and resistance of the work are inevitable, they will twist or bend things a little no matter how practiced you are.
I would have brought Joel's example to octagonal before going into a round swage, I find it's easier to control things with flat planes to look at while working. Or perhaps I should say I can see when something is out of wack earlier with flats over rounds.